Passive thermal images, including millimeter and other wavelengths, provide benefits in practical and safe detection of concealed objects on people. Such techniques are commonly performed by an instrument which creates images from radiometric detector(s). However, one of the main detractors in effective and affordable implementation of such systems is the detrimental effects of gain fluctuation. The effects of gain fluctuation can easily reduce thermal resolution by a factor of two or more, rendering what would otherwise be an effective system almost useless, as explained more fully below.
Various techniques exist for revealing concealed objects. In passive millimeter wave, and other wavelengths in general, imaging for concealed object detection on people, the natural radiation from the person, and the generally lower temperature environment in which the person is immersed, cause a reflective object on that person to reveal itself in contrast because the object reflects a lower temperature than the body radiates. However, in general, these systems employ radiometer(s) that contain electronic amplification and detection components, all of which perform a large amount of signal gain. Technological developments over the years have improved amplifiers so that they operate with a low amount of thermal noise. Such amplifiers, not unlike many electronic components, have gain fluctuation with a spectrum which generally increases in fluctuation amplitude as the frequency decreases. This kind of fluctuation is often referred to as “one over f noise” or simply “1/f” whether it is the spectrum of gain fluctuation, or fluctuation of other electronic functions such as the transfer function of a detector being the ratio of voltage output for power input.
During the period over which a sensor is sampled and employed to form an image, electronic gain fluctuates in amplitude. Those changes in gain result in a change in sensed amplitude or temperature from the scene. These gain-based changes are in addition to both actual thermal changes being sensed and other thermal-like noise sources including electronic ones. As a result, the image is degraded by the amount contributed by gain fluctuations in addition to that which it would otherwise be degraded from thermal noise sources if the gain fluctuations were not present. As the time over which the image is collected increases, the gain fluctuations also increase. In typical existing systems these gain fluctuations compromise performance, and in some cases, especially with long sensing periods, compromise performance significantly, thereby reducing effectiveness of concealed object detection. Therefore, radiometers employed for this purpose are usually fitted with a means for gain fluctuation compensation or removal.
Prior techniques employed for gain fluctuation or removal suffer from 1) added expense of components, 2) loss of signal at sensitive stages of electronic amplification in the case of a switch or significantly increased need for dynamic range in the case of a noise added radiometer, and 3) the loss of sensitivity by a factor of 2 in the cases of a switch or noise added radiometer.
Accordingly, need exists for a system and technique in which passive thermal images may be used for practical and safe detection of concealed objects on people and which does not suffer from the above described negative effects of gain fluctuation introduced by the imaging system during the detection process.